全文获取类型
收费全文 | 2350篇 |
免费 | 300篇 |
出版年
2021年 | 23篇 |
2019年 | 12篇 |
2018年 | 29篇 |
2017年 | 17篇 |
2016年 | 28篇 |
2015年 | 69篇 |
2014年 | 70篇 |
2013年 | 97篇 |
2012年 | 121篇 |
2011年 | 129篇 |
2010年 | 87篇 |
2009年 | 70篇 |
2008年 | 133篇 |
2007年 | 106篇 |
2006年 | 83篇 |
2005年 | 80篇 |
2004年 | 84篇 |
2003年 | 79篇 |
2002年 | 65篇 |
2001年 | 66篇 |
2000年 | 76篇 |
1999年 | 63篇 |
1998年 | 34篇 |
1997年 | 20篇 |
1996年 | 32篇 |
1995年 | 28篇 |
1994年 | 27篇 |
1993年 | 26篇 |
1992年 | 46篇 |
1991年 | 47篇 |
1990年 | 49篇 |
1989年 | 59篇 |
1988年 | 52篇 |
1987年 | 56篇 |
1986年 | 38篇 |
1985年 | 44篇 |
1984年 | 46篇 |
1983年 | 32篇 |
1982年 | 26篇 |
1981年 | 31篇 |
1980年 | 17篇 |
1979年 | 50篇 |
1978年 | 26篇 |
1977年 | 17篇 |
1976年 | 21篇 |
1975年 | 18篇 |
1974年 | 17篇 |
1973年 | 25篇 |
1972年 | 18篇 |
1971年 | 20篇 |
排序方式: 共有2650条查询结果,搜索用时 31 毫秒
121.
A new genus of Devonian age fossil plants is described from the Trout Valley Formation of northern Maine. Abundant compression material permits a rather complete understanding of its morphology. Pertica quadrifaria Kasper and Andrews, gen. et sp. nov., was an erect plant, perhaps a meter tall, with a pseudomonopodial main axis and dichotomous side branches. The side branches were arranged in a clockwise spiral (from base to apex) and were tetrastichous. They dichotomized numerous times, with the intervals between dichotomies decreasing distally. The ultimate branchlets bore numerous sporangia in dense clusters. Other side branches were completely sterile. Pertica quadrifaria is classified in the Subdivision Trimerophytina of Banks. Its evolutionary significance rests in the fact that it is a link in the chain of increasingly complex early vascular land plants. 相似文献
122.
123.
C. T. Andrews 《BMJ (Clinical research ed.)》1957,1(5016):427-433
124.
G P Hess J P Andrews G E Struve 《Biochemical and biophysical research communications》1976,69(3):830-837
The kinetics of acetylcholine receptor-mediated flux of 22sodium ions from microsacs has been measured in the presence of activators (carbamylcholine and decamethonium) and an inhibitor (d-tubocurarine) of neural transmission. The dependence of the first-order rate constant, kobs, for 22sodium ion efflux on either decamethonium or carbamylcholine concentration does not exhibit cooperativity. The apparent cooperativity observed by Kasai and Changeux in dose-response curves for 22sodium flux from the same preparation is adequately accounted for by the contribution which efflux from non-excitable microsacs, the main component of the preparation, makes to the measurements. d-Tubocurarine was found to be a non-competitive inhibitor of decamethonium-activated 22sodium efflux. The results of the kinetic measurements are in agreement with equilibrium measurements of the interaction of decamethonium with the same microsac preparation, i.e. adherence to a classic Langmuir binding isotherm and separate binding sites for activators and inhibitors of neural activity. The results indicate a direct relationship between ligand binding and receptor-mediated ion flux. How these two processes contribute to electrophysiological measurements is not apparent. 相似文献
125.
126.
127.
Amanda R. Goldberg Courtney J. Conway David C. Tank Kimberly R. Andrews Digpal S. Gour Lisette P. Waits 《Ecology and evolution》2020,10(14):7627-7643
In herbivores, survival and reproduction are influenced by quality and quantity of forage, and hence, diet and foraging behavior are the foundation of an herbivore's life history strategy. Given the importance of diet to most herbivores, it is imperative that we know the species of plants they prefer, especially for herbivorous species that are at risk for extinction. However, it is often difficult to identify the diet of small herbivores because: (a) They are difficult to observe, (b) collecting stomach contents requires sacrificing animals, and (c) microhistology requires accurately identifying taxa from partially digested plant fragments and likely overemphasizes less‐digestible taxa. The northern Idaho ground squirrel (Urocitellus brunneus) is federally threatened in the United States under the Endangered Species Act. We used DNA metabarcoding techniques to identify the diet of 188 squirrels at 11 study sites from fecal samples. We identified 42 families, 126 genera, and 120 species of plants in the squirrel's diet. Our use of three gene regions was beneficial because reliance on only one gene region (e.g., only trnL) would have caused us to miss >30% of the taxa in their diet. Northern Idaho ground squirrel diet differed between spring and summer, frequency of many plants in the diet differed from their frequency within their foraging areas (evidence of selective foraging), and several plant genera in their diet were associated with survival. Our results suggest that while these squirrels are generalists (they consume a wide variety of plant species), they are also selective and do not eat plants relative to availability. Consumption of particular genera such as Perideridia may be associated with higher overwinter survival. 相似文献
128.
Julian E. Andrews 《Ichnos》2013,20(4):247-253
The Kilmaluag Formation of the Great Estuarine Group (Middle Jurassic) of Scotland represents deposition of mixed carbonate and clastic sediments in a low‐salinity coastal lagoon to floodplain lake setting. Large, unusual trace fossils occur at two horizons within the formation. One type consists of platelike structures about 50 cm in diameter, which are found on wave‐rippled sandstone. These structures, strikingly similar to burrows produced by modern mudskippers, are assigned to fish that shallowly burrowed into the lagoon‐shore sediment. The second type of burrows, found in brecciated, dolomitic limestones, are pipelike, about 4 to 7 cm in diameter and as much as 50 cm deep. One example has a chamber at the base of the pipe. Although most features of these structures appear similar to modern lungfish burrows, the chamber is most similar to structures produced by modern crayfish. The animal probably burrowed into the moist, mudflat sediment to escape desiccation during seasonal aridity. 相似文献
129.
Andrew E. Schuh Thomas Lauvaux Tristram O. West A. Scott Denning Kenneth J. Davis Natasha Miles Scott Richardson Marek Uliasz Erandathie Lokupitiya Daniel Cooley Arlyn Andrews Stephen Ogle 《Global Change Biology》2013,19(5):1424-1439
An intensive regional research campaign was conducted by the North American Carbon Program (NACP) in 2007 to study the carbon cycle of the highly productive agricultural regions of the Midwestern United States. Forty‐five different associated projects were conducted across five US agencies over the course of nearly a decade involving hundreds of researchers. One of the primary objectives of the intensive campaign was to investigate the ability of atmospheric inversion techniques to use highly calibrated CO2 mixing ratio data to estimate CO2 flux over the major croplands of the United States by comparing the results to an inventory of CO2 fluxes. Statistics from densely monitored crop production, consisting primarily of corn and soybeans, provided the backbone of a well studied bottom‐up inventory flux estimate that was used to evaluate the atmospheric inversion results. Estimates were compared to the inventory from three different inversion systems, representing spatial scales varying from high resolution mesoscale (PSU), to continental (CSU) and global (CarbonTracker), coupled to different transport models and optimization techniques. The inversion‐based mean CO2‐C sink estimates were generally slightly larger, 8–20% for PSU, 10–20% for CSU, and 21% for CarbonTracker, but statistically indistinguishable, from the inventory estimate of 135 TgC. While the comparisons show that the MCI region‐wide C sink is robust across inversion system and spatial scale, only the continental and mesoscale inversions were able to reproduce the spatial patterns within the region. In general, the results demonstrate that inversions can recover CO2 fluxes at sub‐regional scales with a relatively high density of CO2 observations and adequate information on atmospheric transport in the region. 相似文献
130.